Two main types of an energy device are an energy storage device and an energy generating device. Typical examples of the energy storage device are an electrochemical capacitor and a battery, which have already been used in their appropriate markets. Examples of the electrochemical capacitor are: an electric double layer capacitor which uses an activated carbon as a polarizable electrode active material and utilizes an electric double layer formed at an interface between a pore surface of the activated carbon and an electrolytic solution; and a redox capacitor which uses a transition metal oxide, such as ruthenium nitrate, whose valence continuously changes, and an electrically-conductive polymer which can be doped. Moreover, two main types of the battery are: a secondary battery which can be charged and discharged by utilizing intercalation and chemical reactions of active materials; and a primary battery which is basically not rechargeable after being discharged once.
The most basic component common to all of these various energy storage devices is an electrode active material which can discharge energy in principle. To take out the energy stored in the electrode active material, a current collector (electric conductor) is further required, which has electron conductivity and is electrically connected to the electrode active material. Since the current collector needs to transfer the energy of the electrode active material with high efficiency, a metallic material, such as aluminum, copper, or stainless steel, which is very low in resistance is typically used as the current collector. However, in the case of using the electrolytic solution, such as a sulfuric acid aqueous solution, which causes metal to corrode, for example, a rubber-based material to which electrical conductivity is given may be used as the current collector.
As the application of the energy storage device is increasing in recent years, there is a need for the energy storage device which has excellent properties, i.e., which is lower in resistance and can discharge higher current. First, these properties were expected of the electric double layer capacitor which was the lowest in resistance in principle among the energy storage devices, and the electric double layer capacitor having such properties was realized by disposing a carbon-based electrically-conductive layer on a joint surface between the electrode active material and the current collector. Since an electronic resistance in the electrode active material of the electric double layer capacitor is comparatively lower than those of the other secondary batteries, a contact resistance between the electrode active material and the current collector accounts for a nonnegligible percentage with respect to the resistance of the device, so that the carbon-based electrically-conductive layer is disposed on the joint surface. At present, similar technical trend to the above has been pursued for a lithium secondary battery.
To solve the above problems, an energy device has been studied, which uses carbon nanotubes, each having one end connected to the current collector, as the electrode active material (see Patent Document 1, for example). The carbon nanotube is a hollow carbon material having a minimum diameter of 0.4 nm and a maximum length of 4 mm. Unlike conventional pellet electrodes, a carbon nanotube electrode in which one end of each carbon nanotube is connected to a substrate does not require an electric conduction assisting material and a binding material. Therefore, the volume fraction of the active material in the carbon nanotube electrode is 100%. In addition, since the electrode active material and the current collector that is the substrate are connected to each other, the carbon nanotube electrode is very low in electrical resistance. Further, the carbon nanotube has an extremely high ideal specific surface area of 2,625 m2/g, and is especially suitable to be applied to the electric double layer capacitor.
In recent years, it is reported that carbon nanotubes are synthesized at a high growth rate by forming an alumina (aluminum oxide) film as a buffer layer on a silicon substrate, further forming catalyst particles, and then introducing water as an oxidizing agent (see NPLs 1 and 2, for example). After these reports, in most of reported cases in each of which the carbon nanotubes are synthesized at a high growth rate, the alumina (aluminum oxide) film is used as the buffer layer.
In each of NPLs 1 and 2, after an alumina-containing buffer layer is formed on a high melting point substrate, such as a silicon substrate, and the carbon nanotubes are synthesized, only the carbon nanotubes are transferred to an aluminum substrate, and the obtained aluminum substrate is used as the electrode of the energy device.